Data transmissions in a wireless communication system are susceptible to interference and other conditions that can impact radio links, which can result in a data unit being received in error, or even not received at all by a receiver. As such, nodes in a wireless communication system generally implement one or more mechanisms to support retransmission and error correction in the event that a data unit is dropped on the radio link or received in error. For example, some wireless communication systems, such as Long Term Evolution (LTE) systems, implement a mechanism known as hybrid automatic repeat request (HARQ) that can respond to errors by performing retransmission of data at a physical (PHY) layer in response to a retransmission request by the receiving node.
In LTE systems, an evolved node B (eNB) can send an acknowledgement status—e.g., an acknowledgement (ACK) or a negative-acknowledgement (NACK)—for a data unit sent by a wireless communication device served by the eNB on a physical HARQ indicator channel (PHICH). The acknowledgement status sent on the PHICH can be used by the media access control (MAC) layer of the wireless communication device to determine whether to perform a HARQ retransmission. In this regard, if the eNB does not successfully receive the data unit and sends a NACK, the wireless communication device can perform a HARQ retransmission to retransmit the data unit to the eNB. However, the PHICH is susceptible to interference such that HARQ ACKs/NACKs can often be corrupted in transmission and/or otherwise misinterpreted by the wireless communication device. In this regard, if multiple devices use the same frequency and orthogonal code and one transmits data while the other is receiving a HARQ ACK/NACK on the PHICH, the resulting interference can result in the HARQ ACK/NACK being misinterpreted by the device receiving the HARQ ACK/NACK. Accordingly, in some cases, a wireless communication device can needlessly perform a HARQ retransmission where HARQ retransmission was not requested by the eNB, thus slowing down the uplink transmission rate. Alternatively, if a wireless communication device misinterprets a HARQ NACK as an ACK, the device can continue on with transmission of the next data unit, which can result in the previous data unit being dropped at the eNB, thus negatively impacting performance. In this regard, HARQ is not designed to provide 100% transmission accuracy, but rather is designed to provide for fast retransmission to support high throughput applications while satisfying a target error rate, such as a maximum block error rate (BLER) of 10%.
Accordingly, while HARQ is designed to provide for fast retransmission and error correction at the physical layer in fast fading conditions, HARQ is not designed to be fully robust for applications requiring a low error rate. Further, HARQ implementations do not provide for reordering of out of order data packets at the MAC layer. For this purpose, a second retransmission mechanism known as automatic repeat request (ARQ) can be implemented at the radio link control (RLC) layer in LTE systems. Acknowledgement statuses used for ARQ in the RLC layer can be sent to a wireless communication device by an eNB on the physical downlink shared channel (PDSCH), which is less susceptible to interference than the PHICH. Accordingly, the ARQ mechanism implemented by the RLC layer can be more robust to interference and provide a higher level of accuracy than that provided by HARQ at the physical layer. However, ARQ is a slower mechanism that does not result in correction of errors as quickly as HARQ, and retransmission via ARQ processes can reduce throughput more than HARQ retransmission processes.
Moreover, there is no coordination between the MAC layer and RLC layer when performing HARQ and/or ARQ processes. In this regard, the acknowledgement status and ARQ process used at the RLC layer is completely siloed and separate from the acknowledgement status and HARQ process at the MAC layer such that the MAC layer does not have access to or consider any feedback from the RLC layer when determining whether to perform a HARQ retransmission.